The major goals are: 1) to determine the role of viruses of parasitic protozoa in pathologic conditions in humans, 2) definition of genetic factors contained in Entamoeba histolytica which determine host range and/or cell lysis, and 3) confirmation of genetic basis of variation in host range to E. histolytica in the absence of cell mediated immunity and humoral antibody. The immediate goals are: 1) development of serologic tests and immunological definition of the protozoal viruses presently identified, 2) purification of the icosahedral virus(es) which have been associated with all E. histolytica strains studied, 3) identification of group and type specific antigens of the beaded viruses of E. histolytica, 4) determination of the occurrence and possible role of the viruses of E. histolytica in human population groups, 5) characterization of autolytic toxins associated with certain strains of pathogenic amoeba, 6) identification of the mechanism(s) responsible for susceptibility and resistance in avian tissue, 7) development of genetically stable E. histolytica sensitive and resistant chicken flocks and 8) definition of how the genetic constitution of a host may change the amoebae, and how trophozoites may be altered by multiple passages in resistant and sensitive hosts.